Measurement and control of humidity



. May 1946.

C. A. MABEY MEASUREMENT AND CONTROL OF HUMIDITY Filed Aug; 15, 1941 FIG.

DI TY MEASURING Fl 6 3 mwmwrus INVENTOR.

PIC-3.2

} Patented 14, i946 Charles A.{Mabcy; water-burr, Conn., assignor 'to The Bristol poration of Connecticut I Application August 15, 1941, Serial No. 407,01?

v11 Claims.

This invention relates to the determination of humidity, and more especially to an apparatus in which measurement of humidity is determined on the basis of the quantity of dry gas required to adjust to a standard conditiona predetermined quantity of gas of the humidity to be measured.

Methods of humidity measurement depending on changes in physical dimensions. of hygroscopic materials are generally based on the assumption that said changes are a function of per cent relative humidity and that effects of the temperature and pressure of the atmosphere being measured are of a second order of magnitude and may be neglected. It is knowmhowi ever, that such methods are subject to the following limitations:

(a) Therelation between humidity value and I mechanical deflection is essentially non-linear.

(b) The second-order effects of temperature and pressure are not always negligible, and over the ranges of variation met with in practice may have an appreciable bearing on the indications. I,

(c) All hygroscopic materials possess a certain degree of hysteresis, so that the response depends upon the direction from which a given condition is approached;

(d) When such materials are subjected to extremely high, or extremely low, hdity values, or to mechanical strains, they are likely to acquire permanent changes.

.(e) As the response of such materials to a change in conditions follows an essentially ex' ponential law with respect to time, a consider= able. interval is likely to elapse before a condition of-equilibrium with surrounding atmosphere can be completely established.

It is an object of this invention to provide a humidity measuring apparatus which shall be substantially 'free of the above-listed Objections.

It is aiurther object to provide an'apparatus of this class which shall be readily applicable to the control of humidity in atmospheres or moisture content of. materials.

It is proposed, in carrying outthe invention, to make use of a hygroscopic element only as a detector of small humidity changes in a limited atmosphere surrounding it, and to utilize the response of said element to control the proportioning of an admixture of dry gas with the gas whose humidity condition is to be measured, and

Company, Waterbury, Conn, a corment. It can be shown that with such a com bination, if the volume of the unknown gas be 'constant, the humidity condition thereof may I be directly measured by the volume of thedry gas which it is necessary to; add in order to maintain the mixture at a constant humidity.

Experience hasshown that the initial res; 'wnse of many hydroscopic materials to humidity conditions is very rapid, and that the above-mentioned undesirable effects .are minimized or entirely negligible when the atmosphere surrounding the material is permitted to make only small variations from aflxed humidity value.

In the drawing: Fig. 1 is a digrammatic representation, partly in section, illustrating a humidity measuring apparatus embodying the principles of the, in-

vention.

Fig. 2 is a sectional view of an element of the invention, taken on the line a-a in Fig. vl,,.a'nd to'a somewhat enlarged scale.

Fig. 3 is a diagram showing an application of the principles of the invention to the control of moisture content in the, traveling web of. hygroscopic material.

Referring now to the drawing, the numeral l0 indicates a cup-shaped metallic base member having fitted thereto a tube ii having a very thin wall of unglazed porcelain or similar porous or foraminous material, the same being closed at one end and forming with the base member 10 an enclosed chamber l2. A conduit l3 passingthrougha wall of the base member H! ex-,

tends throughout the length of the tube i 5, forum ing means for the admission of dry gas tothe chamber E2. The a conduit !4 also passin through a wall of member! forms an outlet for said gas.

Mounted upon a base plate :5 is a chamber l6 of substantially cylindrical'coniormation and having one end substantially closed by a fiat wall I1 and the other end covered by a screen I8. A conduit is passing through the wall of I chamber "5 provldesffor admission thereto of a stream of gas. Mounted within the chamber i6 is a hygroscopic element 2| responsive to the moisture content'of the atmosphere within the chamber. The element 2i may effectively be of the form shown in section in Fig. 2, consistso maintain a substantially constant humidity) condition in the atmosphere at the sensitive ele-- mg of a member made up of a layer of hygroscopic material 2| such as treated wood, and a layer of non-hygroscopic. material 2|", such as thin flexible metal, said layers being cemented or otherwise secured together to form a com pound strip, and said strip being formed into a helix and mounted upon a' stud 20 attached to the wall l1. By virtue of the material and conformation of said element, the same will respond to changes in the humidity of the atmosphere to which it is exposed, and will tend to deflect in the sense that its free end will travel in a more or less circular arc about the axis of the' helix.

Mounted coaxially with the element 2! and free for rotation about said axis is a spindle 22 attached to said hygroscopic element as by a radial arm 23, and thereby adapted to partake of rotation communicated thereto by changes in humidity. The spindle 22 extends coaxially to the chamber i6 through an opening in the wall I1 and carries a light vane member 24 adapted to swing between opposed orifices of a nozzle member 25, whereby, by virtue of its position with respect to said orifice, the escape of gaseous fluid therefrom may be more or less obstructed. The combination of the vane 24 and the nozzle member 25 constitutes a control couple identical with that fully' set forth and claimed in U. S. Letters Patent No. 1,880,247, issued October 4, 1932, to H. L. Griggs and A. R. Mabey. The member 25 may expediently be joumaled in a pivot bearing 26; and the spindle 22 may be mounted in a pivot bearing 21, both said bearings being disposed coaxially to the hygroscopic element 2|, whereby the vane 24' will be arcuately deflected with changes in the humidity of element 2| and the member 25 may be adjusted with respect to the position of the vane 24.

Mounted upon a base plate 30 is a pilot valve 3| of the supply-and-waste type, having two seat portions 32 and 33, and a seating member 34 adapted altematively to engage said seat portions. The member' 34 rests in a cavity 35 determined by the seat portions 32 and 33. A capsular spring or bellows 31 mounted upon the base plate 30, and adapted to actuate the seating member 34 by means of a stem 38, provides means for varying the-position of said seating member with respect to said seat portions. Coaction between the seating member 34 and the seat porjion 32 controls communication between the cavity 35 and a conduit 39. Coaction between the seating member 34 and the seat portion 33 controls communication between the cavity 35 and an annular vent surrounding stem 38 and leading to the atmosphere.

Gaseous fluid, which may be compressed air or other gas suited to the purpose in hand, is obtainedfrom a pressure source 40 and conducted to the inlets of two reducing valves 4| and 42 of the conventional type, by which there may be maintained at the outlet sides of said valves predetermined and constant pressures lower than the pressure at the inlet side. The outlet side of valve 4| communicates directly with conduit 39, whereby gaseous fluid from said reducing valve may be admitted to said cavity and the pressure therein made subject to regulation, according to.the extent to which the seating member 34, by its position in said cavity with respect to the seat portions 32 and 33, modifies the relative communication of said cavity with the conduit 39 the member 25 provided by the vane 24. In other calcium chloride socdisposed as topermit free passage of gas therethrough in intimate-contact with said material. A conduit 48 leading from the outlet of the drier 41 communicates through a constriction or orifice-plate 49 with the conduit l9 connected to the interior of chamber l6 containing the hygroscopic element 2|. The constriction 49 cooperates with the regulator 3| in maintaining. a steady flow of dry gas into the conduit I9. A flow meter 50, connected in the conduit 48 provides an indication of the rate of flow of gas therethrough. Or, since the gas in the conduit I9 is at all times at substantially atmospheric pressure, a pressure gauge connected to the conduit 48, providing a measure of the pressure drop through the orifice or constriction 49, may be used as a means of determining the rate of flow through said orifice, and hencev from the drier 41 to the conduit I9. From the outlet side of reducing valve 42 a conduit 5| leads to the interior of a drier 52 similar in all respects to the drier 41; and the outlet side of said drier communicates directly with conduit l3 providing for admission of gas to the chamber l2.

The operation of the device is as follows: The porous tube II is placed in the atmosphere whose humidity it is desiredto measure, and by virtue of the property of absorption will assume a moisture condition corresponding to said humidity. The reducing valve 42 is adjusted to regulate an outflow of air or other gas to a very low pressure, just sufiicient to maintain a continuous flow through the drier 52 and the chamber l2 into the chamber l6, containing the hygroscopic element. The stream of gas passing through the chamber 12 and coming into intimate contact with the interior surface of the tube II will have imparted to it a moisture content representative of the moisture condition of the tube a nd therefore of the temperature to which said tube is exposed. This stream of gas will enter the chamber IE but, because of the free communication of said chamber with the atmosphere, will not appreciably increase above atmospheric value the pressure therein, and will tend to aflect' the angular position of the humidity-sensitive ele-- ment 2|, and therefore of the vane 24.

The action of the control system commanded by the position of the vane 24 is fully set forth in the above-mentioned Patent No. 1,880,247.

This action may be briefly recapitulated as follows: gas is delivered at a low pressure (for example, 15 pounds per square inch) from the reducing valve 4| into theconduit 39, and through the constriction 45 to the conduit 44 communicating with capsular spring 31 arid the orifice member 25. By virtue of the constriction 45 the pressure in the conduit 44 and elements directly connected thereto will .depend upon the amount of obstruction to the escape of gas from words, change in the position of the vane 24 with respect to the orifice member 25 will affect the back pressure in the conduit 44, therefore, in the bellows 31. Gas from the conduit 39 will pass the seating element 32 into the cavity 35 and thence into the conduit 46. The amount of flow a,4oo,4a7

lows 88 adapted to actuate a relay valve 68 in vane 24, and therefore of the humidity of the at mosphere within the chamber l6.

' Gas admitted to the conduitf48 through the valve 3| will pass through the drier 41, and thence through the conduit 48 and the constriction 49 to the conduit I9, where it Will be mixed with the stream of gas coming from the chamber 12 through. the conduit l4. The humidity of the gas within the chamber l6 will then represent that of a mixtureof gas having a'humidity value to be measured and gas having a constant, and

extremely low, humidity value. In operation, the. control element is given an initial setting by means of adjustment of the member 25 about its axis of rotation, so-that the pneumatic control sys tern will tend to regulate the flow of dry gas in a manner to maintain the. gaseous mixture in the chamber l6 at a predetermined humidity value. This setting .will depend on the range of humidity in which it is desired to make measurement.

With the sample gas passing through the chamber I6 from the chamber ii, any change in the humidity value of said gas will operate the vane 24, and hence the pilot valve 3|, in a sense to vary the amount of gas supplied through the conduit 48 and the constriction 49 to bring the humidity in the chamber l8 back to the original setting. By virtue of the constriction between the conduits 48 and IS, the static pressure as measured by the gauge 50 will be a function of the flow of gas through the constriction 49 and therefore of the amount of gas required to maintain the humidity in the chamber Ii at the predetermined value. As has already been pointed out, the amount of gas required to mix with the a manner to admit more or less pressure fluid from the source 48, or other convenient source, to a diaphragm actuated valve 81 connected in a pipe 88 adapted to admit steam or other drying medium to the interior of the roll, whereby the moisture content of the web as it leaves said roll may be aflected.

The operation of this device is as follows: the tube ll being juxtaposed to the web 82, will partake of its moisture condition, which being determined by the instrument 64 as hereinbefore explained, will cause to be developed in the conduit 48 a pressure which will be a measure of the moisture content of the web 62. Should the moisture content of b said web rise above a predetermined value, the pressure of the gaseous medium in the conduit 48, being communicated to the capsular spring 85, will cause the relay valve 86 tomodify the pressure applied to the diaphragm 81 in asense to increasethe supply of drying medium through the pipe 68 to the interior oi the roll 80. In a similar manner, should the moisture. content of the web 62 tend to decrease, the flow of drying medium through the conduit 68 will be modified in a senseaifb restore the web 82 to a normal. and predetermined moisture content. Thus, there has been provided sample in order to maintain a substantially constant humidity value within the chamber I8 is a direct measure of the moisture content of said sample. Thus, the reading of the pressure gauge 50 becomes a measure of the humidity of the atmosphere to which the external surface of the been considered as referring only to the amount a of water vapor carried in air or some other gas, it is understood that the term is essentially of much broader application, and should be construed inits most comprehensive sense; For the purposes of this invention, said terms. are to be strument tending to maintain the measured humidity at a predetermined value.

In Fig. 3 is shown anzapplication of the principles of the. invention to control the moisture condition in a web of hygroscopic material, such as paperv or the like, during the course of manufacture.

Between two rolls 80 and BI .is caused to pass may be identical in all respects with that hereinbefore described in conjunction with the tube II in Fig. 1; said apparatus being connected to tube I I through conduits l4 and 53,. and receiving a supply of air or other gaseous medium from a source 48, all of which'hasbeen fully set forth understood as relating to'the presence of any condensible vapor in a gas or in a mixtureof Furthermore humidity," as employed is defined as the ratio, in per cent, of the amount. or partial pressure, of the condensible constituent of a gas or mixture of gases under given conditions of temperature to the maximum amount, or maximum partial pressure, of such condensible constituent possible for satura-' tiin at the given temperature. 1

'The terms and expressions which I have employed are used as terms of descriptionand not of limitation, and I have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various the invention claimed.

I claim:

1. Apparatus for determining the moisture content of a medium, comprising humidityresponsive means, means for supplying to said humidity-responsive means a mixture comprising the first-mentioned medium together with a second medium having a predetermined moisture content, means controlled by said humidityresponsive meansto'regulate the flow of said second medium for maintaining the mixture of said media at a substantially constant and pre and explained. The conduit is extended to inelude a pressure sensitive capsular spring or beldetermined moisture content intermediate that of said first and second media, and means responviding a measurement of the moisture content of said first medium.

2. Apparatus for determining the moisture contentof a medium, comprising an enclosure in free communication with the atmosphere, a humidityeresponsive device in said enclosure, a

second medium at a pressure above atmospheric and having a predetermined moisture content, means for applying to said enclosure to aflect said device a mixture of said media, means controlled by said humidity-responsive means to regulate the flow of said second medium, for maintaining the mixture of said media at a substantially constant and predeterminedmoisture content intermediate that of said first and second media, and means responsive to the flow of said second medium for providing a measurement of the moisture content of said first medium.

3. In combination, means {or feeding a web of material, means for imparting to a medium under pressure a moisture content corresponding with that of said material, means for effectin-g'mixture of a second medium under pressure with said first-named medium, regulating means responsive to the moisture content of said mixture to control the pressure of said second medium, and thereby the proportions of said media in said mixture, and means positioned in response to the magnitude of said pressure.

4. Apparatus for determining the moisture content of a medium, comprising a chamber having a porous wall exposed to said medium and adapted to assume a condition corresponding to said moisture content, means for passing through said chamber a substantially constant flow of a medium of low moisture content to be afiected by Y said condition, a second chamber, means for actuated by said element to control the rate or admission of said similar medium.

'7. Apparatus for determining the moisture content of a medium, comprising a chamberhavinga porous wall exposed to said medium, means supplying two streams of a gaseous medium, means for reducing the medium constituting each of said streams to a state of low moisture content, means for causing one of said streams to pass at a substantially constant rate through said chamber to assume a moisture content. corresponding to the condition of said wall, means for causing the other of said streams to mix with the first-mentioned stream after passage through said chamber, means for regulating the rate of flow of said other stream, means responsive to the moisture condition of the mixture to control said regulating means, and means responsive to the flow of said other stream to provide a measurement as an index of the moisture content of said first-mentioned medium.

8. Apparatus for determining the moisture content of a medium, comprising a chamber having a porous wall exposed to said medium, means supplying two streams of a gaseous medium at pressures above atmospheric, means for reducing the medium constituting each of said streams to a state of low moisture content, an enclosure in free communication with the atmosphere, means for causing one of said streams to pass at a substantially constant rate through said chamber to assume a moisture content corresponding to the condition of said wall and thence to pass into said enclosure, regulating means for causing theother of said streams to pass into said enclosure to mix therein with the first-mentioned stream, to form a'mixture at substantially conducting said afiected medium to said second chamben-means for admitting to said second chamber a further medium for admixture with said aflected medium, together with means sensitive to the moisture content of said mixture for controlling the rate of admission of said further medium, and means responsive to said rate of admission for providing a measurement as an index of the moisture content of said first-mentioned medium.

5. Apparatus for determining the moisture content of a medium, comprising a chamber having a porous wall exposed to said medium, means for passing through said chamber a stream 01 dry gaseous medium, whereby said dry medium may partake of the condition of said wall, means for providing a second stream of said dry medium unaffected by said condition, means for producing a mixture of said streams at substantially atmospheric pressure, a moisture sensitive element exposed to said mixture and regulating means responsive to said moisture sensitive element tor controlling the volume of said second stream, and means responsive to the flow of said second stream for providing a measurement as an index of the moisture content of said firstmentioned medium; v

6. In an apparatus for determining the moisture content 0! a gaseous medium, means providing a substantially continuous stream oi. said medium, means for admitting to said stream a quantity of a similar medium 01 substantially difierent moisture content to produce a mixture "of 'saidmediaa chamber adapted to contain a.

i said first-mentioned medium.

atmospheric pressure, means responsive to the moisture condition of the mixture to control said regulator means, and means responsive to the flow of said other stream for providing" urement as an index of the moisture co 9. Apparatus ior determining the moisture content of a medium, comprising a chamber having a porous wall exposed to said medium, means supplying two streams of a gaseous medium,

means for reducing the medium constituting each of said streams to a state of low moisture content, means for causing one of said streams to pass at a substantially constant rate through said chamber to assume a moisture content corresponding to the condition of said wall, means for causing the other of said streams to mix with the first-mentioned stream after passage through said chamber, means for regulating the rate of I content of a medium; comprising a chamber havmeans for reducing the medium constituting each or said streams to a state of low moisture content, means !or causing one of said streams to pass at a substantially constant rate through said chamber to assume a moisture content corre-' sponding to the condition or said w'alL-means for causing the other of said streams to mixwith the fluid of s humidity to be measured. means first-mentioned stream site: psssele throush a comm-bin: s. reguistor for mducin: a regulated said chamber, means for rezuistins the rate of stream of gaseous fluid of a moisture content less now of said other stresm. meuns responsive to than that of the first-named. fluid, means forthe moisture condition of the to control s producing a mixture of said streams, as hymsaid regulator means to cause said regulator seopic element exposed to said mixture to be instsnt moisture content, tocether with means for regulator to produce it reluisted stream to 'msinmeasurinsjhe rate of supply of said gaseous me- 10 tsin the mixture at s substantially constant hudium in said other stream as an index of the q midity and means res nsive to the 110110! said moisture content of said first-mentioned meresale-ted stream for providinz a measurement dium. v sssnindexoithemoisturecontentofthenrst- 11. In a humidity meter, the combination of mentioned stresm.

means for producing a; constant stream of gase- 5 v v CHAR-LE8 A. MABEY, 

